The present invention relates to a tomogram creating device, a tomogram creating method and a radiation examining apparatus, and particularly to a tomogram creating device, a tomogram creating method and a radiation examining apparatus suitable for application to the creation of tomograms by positron emission CT (Position Emission Computed Tomography (hereinafter called “PET”)) or the creation of tomograms by X-ray CT and single photon emission CT (Single Photon Emission Computed Tomography (hereinafter called “SPECT”)).
As a technology for non-invasively imaging functions, forms in a body of an object to be examined or an examinee, there is known an examination using radiation. As typical ones of radiation examining apparatuses, there are known X-ray CT, MRI, PET, SPECT, etc.
The X-ray CT is a method of irradiating an object to be examined or an examinee with radiation emitted from an X-ray source and imaging the form of the body from transmission of the radiation in the body of the examinee. By detecting the intensity of X rays transmitted through the body by a radiation detecting element, a linear attenuation coefficient between the X-ray source and the radiation detecting element is obtained. The linear attenuation coefficient of each voxel is determined by a Filtered Back Projection Method or the like described in IEEE Transaction on Nuclear Science Vol. NS-21, pp 228-229. The determined value is converted into a CT value. A radiation source well used in the X-ray CT is about 80 keV or so.
The PET examination is a method of administrating a radioactive medical agent (hereinafter called “PET medical agent”) containing positron emission nuclear species (15O, 13N, 11C, 18F, etc.), and a substance (marker substance) having the property of concentrating on a specific cell in a body to an examinee corresponding to a body to be examined, and examining in which region the PET medical agent is consumed or used up in excess. Positrons emitted from the PET medical agent are coupled to electrons in the vicinity thereof to yield positron extinction, thus radiating a pair of γ rays (hereinafter called “γ-ray pair”) having an energy of 511 keV. Since the respective γ rays of each γ-ray pair are radiated in directions directly opposite to each other, in between which two of radiation detectors the positrons are emitted, is found out if the γ rays of each γ-ray pair are detected by the radiation detectors. By detecting a large number of the γ-ray pairs, locations where the PET medical agent is much consumed, can be found out. When a PET medical agent using sugar or glucose as the marker substance is administered to an examinee, for example, a cancer acutely varied in carbohydrate metabolism can be found. Incidentally, the resultant data is converted into data of each voxel in a body by a method such as the aforementioned Filtered Back Projection Method.
In the SPECT examination, a radioactive medical agent (SPECT medical agent) containing single photon emission nuclear species (99Tc, 67Ga, 201Tl, etc.) is administered to an examinee and γ rays emitted from the nuclear species are detected by a γ-ray detector. The energy of the γ rays emitted from the single photo emission nuclear species is a few 100 keV or so. Since the single γ ray is emitted in the case of the SPECT, the angel at which it is launched into the corresponding radiation detector, cannot be obtained. Thus, a collimator is used to detect only γ rays incident from a specific angle by means of the corresponding radiation detector, whereby angular information is obtained. Even in the case of the SPECT, the resultant data is converted into data of each voxel in a body by a method such as the Filtered Back Projection Method or the like. Incidentally, a transmission image is often imaged even in the case of the SPECT. The half-life of the single photon emission nuclear species used in the SPECT is longer than the half-life of the positron emission nuclear species used in the PET and ranges from six hours to three days.
Nuclear medicine examinations of the PET and SPECT need examination time intervals ranging from a few minutes to several tens of minutes, whereas MRI and X-ray CT examinations are completed in a few seconds to several tens of seconds. Therefore, the MRI examination and X-ray CT examination can be carried out in a state in which an examinee is being unbreathed. Since, however, the PET and SPECT or the like are not capable of performing examinations in the unbreathed state, the examinations are performed in a state in which the examinee is being breathed. Therefore, there is adopted a method of reducing artifacts (displacements of an image incident to the motion of a body) produced due to the motion of the body by means of a method such as the collection of only data in a specific state according to breath synchronization.
Japanese Patent No. 3022773 proposes detecting a chest position with breathing of an examinee by means of a position sensor, correcting a SPECT image, based on a signal detected by the sensor and correcting image blurring produced with breath movements.
Since the examination time becomes long in the method of eliminating the breath artifacts according to the breath synchronization, the same method is inefficient. The correcting method described in Japanese Patent No. 3022773 is capable of shortening an examination time interval. Since, however, the correcting method described in Japanese Patent No. 3022773 detects the chest position (breath-based swing) of the examinee by means of the position sensor, the swing of each individual region in the body of the examinee cannot be accurately grasped. Thus, a tomogram good in accuracy cannot be obtained depending on the correction of Japanese Patent No. 3022773, and the accuracy of a diagnosis effected on a region on which a radioactive medical agent is concentrated in the body, cannot be improved.